In late 2022, CVE-2022-41757 was disclosed—a serious security vulnerability in the Arm Mali GPU Kernel Driver that could let a regular, unprivileged user gain write access to protected read-only memory or even access memory that had already been freed. This issue is a classic example of a memory corruption exploit, and, given the popularity of devices using the Mali GPU (think many Android phones and embedded devices), it carries real risk.

Valhall r39p (before r40p fixed it)

If you’re wondering what all that means, don't worry! Let’s break down how it worked, show you some code snippets, and most importantly—explain what you need to do to stay safe.

Why Does This Bug Matter?

The Mali GPU kernel driver runs in ring  (kernel) space, where it has high privileges. A bug here lets *any app*—even a malicious one without root or system-level access—tamper with sensitive memory. In practical terms: it opens the door to privilege escalation, data corruption, and potentially total device compromise.

Technical Breakdown: Memory Corruption in the Mali Driver

When you perform graphics operations, the GPU driver is supposed to make sure user-provided commands don’t do anything dangerous. What happened here is the safeguards in the Mali driver weren’t tight enough.

Here's the key:  
A non-privileged user could trick the GPU into writing to memory they technically shouldn't access—memory that might be marked as read-only, or worse, memory the kernel had *already freed* (known as a use-after-free).

The Problem in Code (Simplified)

While Arm didn't release the exact driver's source for these proprietary blobs, we can piece together a simplified example based on the nature of these bugs:

// Hypothetical vulnerable code in the driver
// 'cmd_buffer' comes from an unprivileged process
void mali_process_command(void *cmd_buffer) {
    struct gpu_buffer *buf = get_buffer(cmd_buffer);

    // Fails to check if buffer is read-only or has been freed already!
    // Problem: attacker-controlled access here
    memcpy(buf->gpu_mem, cmd_buffer->data, cmd_buffer->size);
}

A properly secured driver would strictly check that buf->gpu_mem still points to valid RAM, and that the permissions allow writing. In the affected versions, those checks could fail under certain conditions.

A crafty attacker can

- Acquire write access to memory segments intended to be read-only (for example, system code segments or kernel data structures).
- Re-access memory that has been "freed," leading to *use-after-free* bugs, which are notorious for being leveraged into code execution exploits.

Proof-of-Concept (PoC) Exploit

Because the source code isn’t public, there isn’t a universal “one-size-fits-all” exploit for this bug.

However, the process generally looks like this (in simplified pseudocode)

# This is a conceptual Python PoC using a hypothetical IOCTL interface
import fcntl, mmap, os

KERNEL_FD = os.open("/dev/mali", os.O_RDWR)

# Step 1: Allocate and free GPU buffer
buf = allocate_gpu_buffer()
free_gpu_buffer(buf)

# Step 2: Reuse the buffer ID before it's cleaned up in kernel space
evil_data = b"A"*4096  # Data we want to write to the freed memory
cmd = make_gpu_command(buffer_id=buf, data=evil_data)

# Step 3: Send the malicious command
fcntl.ioctl(KERNEL_FD, GPU_IOCTL_SUBMIT_CMD, cmd)

The real-world exploit would involve filling the GPU’s command queue, recycling buffers, and timing interactions carefully. But the high-level idea is to *reuse* a buffer handle after freeing—triggering the race that corrupts memory.

Who’s At Risk?

Manufacturers and products using Arm Mali GPU Valhall architecture (r29p–r39p) are affected. This means:

Fixes and Mitigations

- Arm's official fix was released in versions r38p2 and r40p, which tightened permission checks and improved buffer management.
- Check with your device vendor to see if they’ve pushed these driver updates into your Android kernel. Pixel phones and higher-end Samsungs update drivers frequently; lesser-known devices may lag behind.

> 👉 If you’re an OEM or developer:  
> Apply the patches from Arm’s developer page.
>
> 👉 If you’re a user:  
> Keep your phone and apps up to date. Regular system updates often include driver security patches.

References & Further Reading

- Official CVE page: CVE-2022-41757
- Arm Security Bulletin (December 2022)

Final Thoughts

CVE-2022-41757 is a textbook reminder: kernel drivers matter for your security just as much as app updates. The bug's seriousness means you should check if your device has taken the latest Mali driver patch—especially if you're on an Android device that uses these popular GPUs.

In simple terms, this bug turned the forbidden areas of memory into fair game for attackers. If you're a developer or a security pro, make sure your devices are running an unaffected driver version. If you're a regular user, keep those security updates coming!

Timeline

Published on: 11/08/2022 15:15:00 UTC
Last modified on: 11/09/2022 17:21:00 UTC